Since an electro-luminescence device can emit light itself, it is capable of providing a brighter and sharper display than liquid crystal device. Thus, the electro-luminescence device has long been studied by many scholars. Hitherto, an electro-luminescence device comprising an inorganic material ZnS has been made practicable. However, such an inorganic electro-luminescence device requires an applied voltage of not lower than 200 V to emit light and thus cannot find wide application.
On the other hand, the organic electro-luminescence device comprising an organic material, though having heretofore been far from practicable, can enjoy rapid progress in its properties made by a laminated structure developed by C. W. Tang et al. of Kodak Corporation in 1987. They developed a laminate of an organic fluorescent substance, an organic material capable of transporting carriers (carrier transporting layer) and an electrode. Both holes and electrons injected from the respective electrodes were injected into the fluorescent substance, resulting in a successful emission of light. Thus, the light emission efficiency of the organic EL device was enhanced. Many scholars have made studies to enhance the properties of organic EL device, and at present, a luminescence of not lower than 10,000 cd/m.sup.2 can be obtained.
As the organic fluorescent substance which can be used for an organic EL device having a laminate structure there has been used a fluorescent organic dye such as tris-8-quinolinol aluminum complex (Alq) and coumarin. As the carrier-transporting material there have been studied various compounds well known as organic materials for electrophotographic photoreceptor. Examples of such compounds include diamine compounds such as N,N'-di(m-tolyl)-N,N'-diphenylbenzidine (TPD) and 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane (TPAC), and hydrazone compounds such as 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone. Further, porphyrin compounds such as copper phthalocyanine may be used.
The basic luminescence of such an organic EL device is high enough to make the product practicable. The impracticability of the organic EL device is mainly responsible for (1) the lack of stability luminescence on operation, and (2) the lack of storage stability. The deterioration in operation as used herein means the drop of luminance, occurrence of a region which emits no light, i.e., dark spot, or destruction due to device shortcircuit during operation where an electric current is applied to the device. The storage stability as used herein means the stability of luminescence during the storage of the device.
In order to eliminate these difficulties of organic EL devices in luminescence stability and storage stability, the inventors have made studies of the mechanism of deterioration of organic EL devices. As a result, it was found that the deterioration of the properties of organic EL devices is mainly responsible for the properties of a carrier-transporting layer. In some detail, it was found that a commonly used hole-transporting material such as those described above (1) crystallizes due to moisture, temperature or current to give unevenly shaped thin film, (2) denatures with the passage of current, or (3) deteriorates its adhesive property to a substrate and a light-emitting layer, causing a remarkable deterioration of the luminescence of the organic EL device.